Phase-locked loop circuits are commonly used in data communication systems to recover the original clock information and the original stream of binary data from Manchester-coded data, and such circuits are described in U.S. Pat. No. 4,565,976 ('976) that utilizes an arrangement of two voltage-controlled oscillators and U.S. Pat. No. 4,546,486 ('486) that utilizes edge-detecting circuits; both of which U.S. Patents are herein incorporated by reference. The phase-locked loop circuits commonly comprise one or more oscillators, serving as local clocks, whose output phase and/or frequency are "steered" to keep them in sync (lock on) with the received Manchester-coded data.
Manchester-coded data comprising a plurality of binary bits, commonly referred to as data bit cells each having a known period, may be defined herein with reference to FIG. 1. As seen in FIG. 1, a logic "1" is characterized by the transition from a "0" (logic "low" level) to a logic "1" (logic "high" level), in the middle of a data bit cell. A logic "0" is defined by a transition from a "1" (logic "high" level) to a logic "0" (logic "low" level) in the middle of a data bit cell. As further used herein, a logic "0" will be simply referred to as a "0" and, similarly, a logic "1" will be simply referred to as a "1."Manchester-coded data transitions can occur at both the edges of the data bit cells and in the middle of data bit cells. These transients may be difficult to distinguish especially in the presence of noise. More particularly, the signal paths carrying the "1's" and "0's" of the Manchester-coded data may be subjected to noise transients which could possibly be interpreted as data, especially if edge-detection circuits are used in the decoding circuit. Furthermore, these noise transients may occur throughout the data communication system employing Manchester-coded data. If such difficulties are experienced, the particular phase-locked loop circuit may be unable to lock its local clock onto the Manchester-coded data and, thus, be unable to satisfactorily recover the original clock information and the original stream of binary data from the Manchester-coded data transmissions.